Many forms of cancer typically respond to initial treatment. However, some cancers, such as multiple myeloma, are not cured by chemotherapy, and invariably drug resistance emerges (Dalton et al., 1992; Kyle et al., 1982). Traditional in vitro unicellular models of melphalan resistance have identified several acquired melphalan resistance mechanisms including: 1) reduced drug uptake 2) reduced DNA damage, and 3) changes in glutathione levels (Dalton et al., 1992; Bellamy et al., 1991; Gottesman et al., 2002). However, it is currently unclear if these mechanism(s) play a causative role in clinical drug resistance. Moreover, it is not known if drug resistance mechanisms identified following chronic drug exposure (acquired drug resistance) allow for tumor cell survival following initial drug treatment (de novo drug resistance).
Evidence supporting the importance of understanding the influence of the tumor microenvironment on drug sensitivity has been reported by Teicher et al. (1990). These investigators showed that in vivo selection of EMT-6 cells with alkylating agents, results in a drug-resistant phenotype that is operative only in vivo. The tumor microenvironment consists of soluble factors (cytokines), as well as, cell surface receptors (cell adhesion molecules) both of which can influence cellular fate following cytotoxic exposure. More recently, it has been shown that adhesion of tumor cell lines to fibronectin (FN) via β1 integrins contributes to a reversible, de novo drug resistance termed “cell adhesion mediated drug resistance or CAM-DR” (Damiano et al., 1999; Sethi et al., 1999). Adhesion via β1 integrins is known to activate a network of signal transduction pathways that influence cell survival, growth and differentiation (Hanks et al., 1992; Lin et al., 1997; Meng et al., 1998; Meredith et al., 1993). Although the signaling pathway(s) causative for drug resistance have not been entirely delineated, several intracellular targets have been identified that are influenced by β1 integrin adhesion and may contribute to inhibition of programmed cell death induced by either cytotoxic drugs or cell surface death receptors (e.g., CD95). These targets include the following: alterations in the nuclear pool of topo IIβ, increased p27kip1 levels, and changes in the availability of Flip1 binding to FADD (Hazlehurst et al., 2000a; Hazlehurst et al., 2001; Shain et al., 2002). All of these changes occur before toxic or stressful insult.
Interstrand cross-links (ICL) are amongst the most toxic types of DNA damages; therefore, DNA cross-linking agents are important drugs in cancer treatment (Dronkert and Kanaar, 2001). Melphalan, a DNA crosslinker, is one of the most widely used and effective drugs in the treatment of multiple myeloma (MM). Unfortunately, although most patients respond to standard and high dose melphalan therapy, eventually patients will acquire drug resistance. Acquired melphalan resistance is associated with reduced DNA crosslinks, elevated levels of glutathione and increased radiation survival (Bellamy et al., 1991).
Nine Fanconi anemia (FANC) associated genes (FANCA, B, C, D1, D2, E, F, G, and L (Meetei et al., 2003; Meetei et al., 2004a; Meetei et al, 2004b) have recently been found to interact with well-known DNA-damage-response proteins, including BRCA1, ATR and NBS1 (D'Andrea and Grompe, 2003; Pichierri and Rosselli, 2004a; Pichierri and Rosselli, 2004b; Rosselli et al., 2003). The FANC/BRCA pathway plays an important role in the down stream repair of ICL: homologous recombination repair during S phase (Rothfuss and Grompe, 2004). Disruption of the FANC/BRCA pathway results in chromosome instability and hypersensitivity to DNA crosslinking agents.
Pu and Bezwoda (1999) demonstrated that rate of removal of ICL was associated with melphalan resistance in leukemic cells. In another study, Spanswick et al., (2002) showed a similar result in primary Multiple Myleoma specimens. These investigators compared the formation and repair of ICL in plasma cells from melphalan-naive and treated myeloma patients, and found that in vitro sensitivity to melphalan in plasma cells correlated with ICL repair capacity.
As can be understood from the above, there remains a need in the art for preventing or inhibiting the development of drug resistance in the treatment of an oncological disorder.